The vapour space of two 100-m³ heavy fuel oil tanks, both built with stationary and nonflexible roofs, exploded inside a barrack. Welds on the shell/bottom junction were torn apart. The first tank, containing 7 m³ of hydrocarbon, loosened and fell back onto the boiler room. The second (51 m³ of heavy fuel oil) was shot like a projectile 50 m high and fell outside. The ignited fuel oil caused the fire to spread. Broken glass or melted plastic and flying metal debris were observed beyond the barracks’ enclosure. Two explosion sources were identified: a hotspot on the exposed resistances (construction flaw), and flames spewed during the first explosion due to a tank vent installed without a flame arrestor (design flaw).